1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a data transmission and reception method with handover in an Orthogonal Frequency Division Multiplexing (OFDM) communication system.
2. Description of the Related Art
OFDM schemes are especially suitable for high-speed data transmission. With the OFDM scheme, serial input data is converted into parallel streams as based on the number of subcarriers that are modulated, such that the symbol duration can be elongated in proportion with the number of subcarriers while maintaining the original date rate. Because subcarriers are orthogonal to each other, bandwidth efficiency of the OFDM scheme is excellent compared to the conventional Frequency Division Multiplexing (FDM) scheme. In addition, because the symbol durations are longer in the OFDM scheme, the OFDM scheme is more robust against inter-symbol interference (ISI) than a single-carrier modulating scheme.
In general, modulation/demodulation of OFDM signals is executed efficiently using Inverse Fast Fourier Transform/Fast Fourier Transform (IFFT/FFT) or Inverse Discrete Cosine Transform/Discrete Cosine Transform (IDCT/DCT). However, since data modulated using IFFT in a modulation process can be restored to its original form by FFT at the reception side, the number of FFT physical layer modules should be equal to the number of base stations to enable simultaneous reception of data from base stations at different locations. In other words, since a mobile node (MN) needs a simultaneous connection to two base stations to perform soft handover, the MN includes two physical layer modules to perform FFT on the data from the two base stations.
Handovers can be roughly divided into hard handovers and soft handovers.
Hard handovers can be performed using one resource among wireless channels because the MN makes a connection to a new base station in a handover process after cutting off a connection to a previous base station. If the hard handover is used in an OFDM wideband transmission scheme, the handover can be performed using one physical layer module. However, since the currently connected channel is disconnected before handover to the new base station, quality of service (QoS) cannot be guaranteed. Because one data channel and one control channel are occupied in the hard handover, the switching time is long, and since the handover window is designed large to prevent a ping-pong effect between base stations, high power is consumed to transmit data, which increases inter channel interference (ICI). Further, because of the physical channel disconnection effect, the hard handover is not suitable for real-time services.
Unlike the hard handover, in the soft handover, an MN can be simultaneously connected to a plurality of base stations if pilot signal size is within a predetermined window. The MN selects data received from a base station of which a pilot signal has a higher signal-to-noise ratio (SNR). The soft handover provides better QoS guarantee in low-speed real-time data transmission services such as a voice service. However, because two data channels and two control channels are used to perform the soft handover, interference from use of two data channels may occur. Accordingly, the soft handover is suitable for low-speed real-time data such as voice. However, the soft handover has limitations in supporting high-speed data services such as multimedia services, e.g., video on demand (VOD).
FIG. 1 is a diagram illustrating handover in a conventional Universal Mobile Telecommunication System (UMTS) system. Referring to FIG. 1, the UMTS system includes node-Bs 103a, 103b, 103c, and 103d, one a base station, and radio network controllers (RNCs) 105a and 105b controlling the node-Bs 103a, 103b, 103c, and 103d. A UMTS Terrestrial Radio Access Network (UTRAN) 107 composed of the RNCs 105a and 105b is connected to a core network (CN) 109 using an Iu interface. Each of the RNCs 105a and 105b selects a node-B 103a, 103b, 103c, or 103d with a higher SNR using an MN 101 and designated channels.
In FIG. 1, the MN 101 receives packets from the main node-B 103c. As the MN 101 moves closer to the cell border between the main node-B 103c and the sub node-B 103b, the MN 101 senses a pilot signal from the sub node-B 103b getting stronger. When the pilot signal received from the sub node-B 103b exceeds a predetermined threshold, the MN 101 receives packets from the main node-B 103c and the sub node-B 103b simultaneously.
The handover algorithm resides in the RNCs 105a and 105b and the MN 101, and is achieved by signaling between the MN 101 and the RNCs 105a and 105b. The node-Bs 103a, 103b, 103c, and 103d act as a bridge for transmitting signals from the RNCs 105a and 105b to the MN 101.
Since data is modulated/demodulated using IFFT/FFT in an OFDM data transmission/reception scheme, MNs commonly uses two physical layer modules to receive data from different radio access routers (RARs). A system for supporting mobility of an MN using two physical layer modules is disclosed in International Publication No. WO 03/017689.
FIG. 2 is a block diagram illustrating a multiple connection method for supporting mobility of an MN in a conventional OFDM system. FIG. 3 is a block diagram illustrating an MN structure for multiple connections.
Referring to FIG. 2, an MN 302 maintains connections 410 and 414 with first and second base stations 304 and 306, respectively. The connections 410 and 414 include uploading control links 408 and 412, downloading control links 409 and 413, uploading data links 416 and 418, and downloading data links 417 and 419, respectively. Referring to FIG. 3, in order to maintain connections with two base stations, an MN 900 includes an analog processing module 902, an analog-to-digital (A/D) converter 904, a copy module 906, a pair of signal separating circuits 905 and 907, a pair of synchronization loops 908 and 909, and a pair of main digital processing modules 912 and 914.
It is readily apparent that conventional MNs supporting multiple connections, because two physical layer modules are used to support mobility have more complex hardware, increasing manufacturing costs.